1. Field of the Invention
The present invention relates to an immersion microscope objective lens, and more particularly to a mid- and high-magnification ultra-apochromatic immersion microscope objective lens which is used in a near ultraviolet to a near infrared region.
2. Background of the Related Art
The following configuration may be used in a high-magnification immersion microscope objective lens: A meniscus lens having a convex surface facing an image that is cemented with a plano-convex lens having a relatively lower refractive index than the meniscus lens such that the plane of the plano-convex lens faces an object. The cemented lens is disposed closest to the object, and using the difference of the refractive indices of these two lenses, negative refractive power is applied to the cemented surface so as to decrease the Petzval sum in order to correct the curvature of field. Another known configuration is one where two concave surfaces face each other and are disposed at positions where the beam diameter is small. Accordingly, the Petzval sum is decreased by strong negative refractive power thereof, that is, a Gaussian type lens system is created. A combination of both of these configurations is a known technology to effectively decrease the curvature of field.
Known examples of the immersion microscope objective lens having such a configuration are high magnification apochromatic microscope objective lenses disclosed in Japanese Patent Application Laid-Open Nos. H7-230038, 2000-35541, and 2002-148519. In the immersion microscope objective lenses disclosed in these publications, a surface contacting the immersion liquid of a tip lens which constitutes the cemented lens (embedded lens, that is, a lens where a small diameter lens is embedded at the tip) disposed closest to an object, is a plane. The operation to remove bubbles and foreign substances in the immersion liquid is easy, and processing the lens itself is relatively easy.
If the immersion liquid is, for example, oil, the refractive index of the immersion liquid and the refractive index of the tip lens of the embedded lens contacting the immersion liquid can roughly be the same (refractive indices of the oil and glass are both about 1.5). Therefore, even if the surface of the embedded lens contacting the immersion liquid (called a first lens surface) is a plane and the incident angle of the incident light exceeds 60°, the aberrations generated on this plane are small, and a dimensional error of the first lens surface and the center thickness hardly influence the aberrations.
The refractive index of water is about 1.33, which is low compared with the lens (glass). Therefore, if the immersion liquid is water the refractive index difference from the lens generates a spherical aberration on the boundary surface. In particular, when a lens where the NA (Numerical Aperture) exceeds 1.2 and the field of view is wide is designed, the generation of a high order spherical aberration increases and correction of the subsequent lens groups becomes difficult. In fact, if the respective index difference of the immersion liquid and the lens (water and glass) is greater than 0.1, and the first lens surface is a plane of which the NA exceeds 1.2, the correction in the spherical aberration is very difficult. One way to improve this is disclosed in Japanese Patent Application Laid-Open No. 2005-266131, which increases the NA of the objective lens to be greater than 1.2 thereby making the first lens surface contacting the immersion liquid a shallow concave surface.